The invention relates to an oxide cathode comprisimg a base which consists at least substantially of titanium and an aluminium oxide-coated heater element to heat the base, an alkaline earth metal oxide-containing porous layer being provided on the base.
Such an oxide cathode is known from the specification of European Patent Application No. 0059491 laid open to public inspection the subject matter of which specification is considered to be incorporarted in the present specification. In the above Application it is remarked that aluminium oxide is used in most cases for the electrical insulation between the heater element and the base, but aluminium oxide, is not chemically stable in contact with titanium, as a result of which insulation problems may start to occur during the life of the cathode. From the point of view of stability and other thermal and electrical properties, berylium oxide is a very suitable insulation material. A disadvantage, however, is that it is very poisonous. Another suitable insulation material is yttrium oxide, but this is much more difficult to provide on a tungsten heater element than aluminium oxide.